The last Diablo game was released six years ago so you can see why the fans are so hyped out about the possibility that a new game will be announced soon.
While Blizzard has not announced anything yet, an article from GameStar that announced the new Diablo artbook mentioned that the publication will include artwork from Diablo, Diablo II, Diablo III, and Diablo IV. The fact that Diablo IV was mention here has led some fans to believe that a new Diablo game is in the game. Of course, it could be an error as well but most poeple think that it is not.
With that, there was some speculation that Blizzard could be announcing the new Diablo game at BlizzCon which will be happening on the 1st of November and will end on the 2nd of November. We are hoping that this is a cast as many fans were still upset that Blizzard chooses to announce a mobile game last year after all the hype.